This change builds on 0a357ad which supported non-constant strides in
howFarToZero, but used only context insensitive reasoning.
This change does two things:
1) Directly use context sensitive queries to prove facts established
before the loop. Note that we technically only need facts known
at the latch, but using facts known on entry is a conservative
approximation which will cover most everything.
2) For the non-zero check, we can usually prove non-zero from the
finite assumption implied by mustprogress. This eliminates the
need to do the context sensitive query in the common case.
04cd069062